The problem with all definitions of the value of Gold is in trying to measure the value of gold in terms of something else.
Gold is the denominator to measure value of everything else. That is why you cannot establish a value of gold because gold is value. You may as well say "what is the value of value?" or the question posed, reworded, "how do measure the value of value?" Rather, the true subjective nature of the question is rather, how do you determine what you will spend your value (gold) on? The answer to this is a varied as the men who are asked the question.
Everyone measures value in dollars or other currency, but they fluctuate more than gold in the sense that they are printed and increased far more easily and quickly than the supply of gold, or silver for that matter, can be.
Metrology tries to establish the measurement of 7 metrics - length, time, mass, temperature, current, lumens, substance in terms of physical constants. Gold is the physical constant of value. Hence a gold "standard".
If an inch was not an inch, or a meter a meter, how could we agree on length in a contract? You may as well rephrase the question at hand as: "how do you measure the length of an inch?" or "how do you measure the time of a minute?" or "how heavy is an ounce?"
We need standard units of measurement that do not change.
Gold, in that sense, does not change. The increase of supply at less than 2% is on par with both the increase in world population as well as the consumption of gold in industry where it is no longer retrievable.
Credit can explode in derivatives, but Gold remains a physical constant, despite the machinations of alchemists.
If you measure price in ounces or grams of gold, you would see Sine waves in value of commodities over natural cycles versus the ever increasing price in dollars or other currencies, as these currencies are perpetually debased, with varying times from increase in currency supply to the eventual effect on price in said currencies. Ripples may move quickly or slowly through the pool of value.
The reason the value measure in commodities change over time is due to man's estimate of their value in the moment as to need and scarcity. So Gold has not changed, but the desired commodity vacillates in its perceived need and scarcity relative to gold. Commodities can be measured in this fashion to other commodities, but they lack the uniformity, consistency and familiarity and fungibility of gold which make it a useful yardstick.
With reliable measures of value, business and society can prosper as better estimates and projections can be made and transactions can be made more confidently.
In metrology, measuring sticks must be traceably calibrated against standard tools of measurement, which must further be traced to primary standards based on physical constants, such as units of wavelength of light that have low measures of uncertainty.
That which is constant is that which can be relied upon, can be trusted. Man's transactions are built on trust; where trust is compromised, business is undermined.
Gold is incorruptible among elements, and therefore the best measure of value over time. Wood (paper), hay (food commodities) and stubble all burn. Gold is refined by fire and sustains value through the harshest environments, saltwater, floods, humidity, and turbulence of time and mankind's influence.
So the real question is not "how do you measure the value of gold?" but rather "why do you measure value of anything in anything other than gold?" The answer may surprise you, because governments cannot manipulate gold in a free market. Follow the money. Trace it back to gold. Gold limits government in its unchanging value and rarity. They prefer you measure value in their diktat; and by law they make it easier and even necessary to measure value in their creation. But their diktat holds value until it doesn't; when confidence in their con is gone. While every fiat currency eventually reverts to its intrinsic value of zero, or the value of a used piece of paper with negligible heat value in BTUs, Gold remains.
Do not measure the value of gold, measure value in gold.